Most laws of physics, and in particular those that govern acoustics, are symmetrical relative to time. A time-reversed wave propagates like a forward wave going back in time.
Time reversal is a technique for focusing waves, typically acoustic waves, that relies on the invariance of the wave equation on time reversal. A short pulse transmitted from a source point propagates in an unknown propagation medium. A portion of this wave is picked up, generally by a set of sensors known as a time reversal mirror (TRM), digitized, and time reversed before being sent back in the propagation medium. The wave then retraces its previous steps and converges toward the source point, where it forms a short pulse. The signal collected at the source point is in a form virtually identical to that of the original signal transmitted from the source point. In particular, the more complex the propagation medium, the more accurately the reversed wave converges toward the source point.
The time reversal technique can be extended to radiocommunications networks to improve the equalization of the propagation channels and thus the processing of symbols received via those propagation channels. In this situation, using time reversal requires the transmitter to have knowledge about the propagation channels.
In the prior art, no radiocommunications network based on time-division duplexing, referred to more simply as time duplexing, uses the time reversal technique to improve the quality of service both in the uplink direction and in the downlink direction without recourse to additional network resources.